1. Field of Invention
This invention pertains to the field of gamma ray detection. More specifically, the invention relates to a method for determining and correcting the baseline of a continuously sampled signal for use in positron emission tomography.
2. Description of the Related Art
In the field of positron emission tomography (PET), it is well known that to measure the energy absorbed from a gamma ray interacting in a scintillating crystal, the total light from a crystal must be determined by integrating the photomultiplier tube (PMT) current. This current signal represents the rate of light collected by the sensing PMTs or photodiodes. The integration to determine the total light is traditionally performed using analog circuitry via a gated integrator. This method is graphically illustrated in FIG. 1. Shown are the PMT current signal i(t) from a scintillation event and the integration of the current signal i(t), or:e(t)=∫0ti(t)dtIt will be understood by those skilled in the art that either voltage signals or current signals may be measured.
Alternately, as graphically illustrated in FIG. 2, the integration may be performed by using a uniformly weighted summation of digital samples of the signal. In this method, the level of the signal at time t(0) is precisely at zero volts. However, when the PMT signal is AC-coupled to an analog-to-digital converter (ADC), the level at time t(0) is not zero volts, but varies with the count rate and electronic offset errors.
A gamma ray from an annihilation event interacts with a scintillator crystal, which produces a light output sensed by a PMT. For pulse applications, it is advantageous to use positive bias at the PMT, which results in high voltage bias applied to the anode contact of the PMT. To isolate the processing electronics, AC coupling is required between the PMT and the preamplifier stage. AC coupling between multistages reduces the DC offset errors that accumulate throughout the data processing chain. In the schematic illustration of FIG. 3, an isolation capacitor 102 is disposed between the PMT 100 and a pre-amplifier 104 because of the high voltage bias of the PMT 100. Isolation capacitors 102 are also disposed between each output of the pre-amplifier 104 and the inputs of an amplifier 106 in order to reduce DC offset errors.
Although AC coupling is effective in isolating the high voltage PMT signals from the low voltage processing electronics, the average signal level at the input of the ADCs 108 is dependent on the count rate through the isolation capacitors 102 due to charge buildup. For example, FIG. 4 illustrates a baseline shift due to charge buildup in isolating capacitors. Since the differential pulse height for a mono-energy input is repeatable, the change in average value, the common mode level, with count rate results in an error in pulse height measurement. Because position normalization is also dependent on pulse height measurement, final crystal position used to localize the annihilation is ultimately affected.
It is necessary to determine the baseline prior to an event and correct the baseline to a fixed level. Traditionally, this has been performed by using analog negative feedback baseline correction schemes which correct the baseline when a pulse has not been detected by evidence that a constant fraction discriminator (CFD) has not fired. However, with such a scheme it is possible for the CFD to not register an event if the energy of the pulse is low enough not to trigger the CFD. This results in the negative feedback of the analog baseline circuits attempting to incorrectly adjust the baseline since the error signal used to correct the baseline is derived from an event and not the desired average value.
Other methods have been developed to overcome these and similar problems associated with energy measurement associated with a crystal scintillation event. Typical of the art are those devices disclosed in the following U.S. Patents:
U.S. Patent No.Inventor(s)Issue Date5,585,637Bertelsen et al.Dec. 17, 19965,608,221Bertelsen et al.Mar. 4, 19975,841,140McCroskey et al.Nov. 24, 19986,072,177McCroskey et al.Jun. 6, 20006,160,259Petrillo et al.Dec. 12, 20006,252,232McDaniel et al.Jun. 26, 20016,255,655McCroskey et al.Jul. 3, 20016,291,825Scharf et al.Sep. 18, 2001Also of interest is Takahashi, et al., in “A New Pulse Height Analysis System Based on Fast ADC Digitizing Technique,” Conference Record of the Nuclear Science Symposium & Medical Imaging Conference, 1992, Vol. 1, pp. 350-352.
Of these patents, the '140, '177 and 655 patents issued to McCroskey et al., disclose a gamma camera modified to perform PET and Single Photon Emission Computed Tomography (SPECT) studies. These devices utilize SPECT electronics to generate triggering pulse signals for photons indicative of a positron annihilation event which are corrected, on a bundled basis, for position, linearity and uniformity by the same digital processors used by the camera for SPECT studies. While these patents specifically set forth methods to correct for timing delays, McCroskey et al., do not address baseline correction of DC and count rate offsets.
Petrillo et al., in the '259 patent, and Scharf et al., in the '825 patent, disclose a method and apparatus for selectively integrating PMT channel signals in a gamma camera system. In the '259 method, a trigger word is decoded to determine which of multiple PMT channels are affected by a given scintillation event. When two scintillation events overlap both spatially and temporally, only those channels which are affected by both events stop integrating in response to the second event. Pre-pulse pile-up is corrected by removing the tail of a preceding pulse from a current pulse using an approximation of the tail of the preceding pulse based upon the instantaneous energy of the current pulse and the current count rate.
In the '232 patent issued to McDaniel et al., a detector is disclosed as including opposed detector heads having anode signal processors. The anode signal processors perform a sliding box car integration of each PMT anode signal, as well as correct for baseline shifts and pileup from the tails of previous events, vary the length of the box car based on the time between events, and use a peak detection circuit to reduce the dependence of the integrated value on timing differences between the asynchronous events and the synchronous ADC conversion.
Takahashi, et al., discuss a digitizing system using a pulse height analysis system in nuclear spectroscopy, concluding that a technique disclosed therein has a possibility to analyze individual signals with required accuracies and to be used as an advanced signal processing method. It was noted by the authors that one problem is that noted in the present disclosure—that it was often observed that the baseline of the preamplifier output changed greatly due to the tail of the previous pulse. In order to estimate the baseline value under the signal pulse, an averaging method was employed wherein M points of sampled data are summed and averaged. In order to accomplish this method, the summed data is averaged with equally weighted coefficients.